System for enhanced astigmatic focus signal detection

ABSTRACT

An auto-focus system for a DVD reader or other optical access unit which reads and/or writes information from/to at least two information layers on an optical storage medium such as a DVD or DVR. The auto-focus system includes an astigmatic lens. A confocal aperture is placed at the first focal line image generated by the astigmatic lens from light rays reflected from the addressed information layer. The placement of the confocal aperture allows extraneous out-of-focus light rays reflected from non-addressed information layers to be blocked from reaching the photo detector array which then creates a focus error signal (FES). The FES drives a servo assembly connected to an access head of the optical access unit, bringing the access head into focus. In some embodiments, the confocal aperture is “bow-tie” shaped, allowing for simplified assembly and easing in-plane rotational alignment.

BACKGROUND OF THE INVENTION

This invention relates generally to optical storage. Particularly, thisinvention relates to a system providing enhanced astigmatic focus signaldetection using a confocal aperture.

In October 2000, a study released out of the University of California,Berkeley reported that it took the entire history of humankind until theyear 1999 to accumulate 12 exabytes of information (where 1 exabyteequals 1,000,000,000,000,000,000 bytes), which is 50,000 times the sizeof the Library of Congress. The study estimates that this amount ofaccumulated information will double by the middle of the year 2002. Theincredible recent growth in the production of information has made thedemand for information management tremendous.

Over the course of computing history, various methods of informationstorage have been used. Currently, optical storage is particularlypopular, both because it is a removable storage means (in contrast tohard disk drives) and because it offers high capacity at a reasonablecost. Of course, as with many industries, competition is focused onthree factors: bigger, faster, and cheaper. With respect to“bigger,”optical disk manufacturers strive to provide higher capacity disks.Currently, an everyday CD-ROM or CD-RW can store about 600 megabytes ofinformation while a single-sided DVD traditionally can store 4,700megabytes (i.e., 4.7 GB). However, the recent arrival of dual-layeroptical technology was a jump for disk capacity. By writing and readingdata encoded on two different depths or layers of the disk, capacityessentially doubled. For example, a dual-layer DVD can store 8,500megabytes (i.e., 8.5 GB) per side. U.S. Pat. No. 5,993,930 to Hector etal. teaches one approach to a dual-layer optical medium having amulti-layered spacer layer between reflecting layers.

Multi-layer optical systems have new problems not necessarily present inthe standard single-layer systems. For example, in a single layersystem, the access head must be focused, and remain focused, on the soleinformation-carrying layer. Unfortunately, optical disks are notperfectly flat. As the disk spins during operation, the disk can tilt.These and other aberrations result in the information layer varying indistance from the access head and thus causing a need for the opticalsystem to perform auto-focusing corrections. There are a handful ofstandard approaches to providing auto-focusing functionality to anoptical storage system, including the astigmatic, Foucault, criticalangle method, and differential methods. Of this group, the astigmaticmethod is perhaps most widely used.

In an astigmatic auto-focusing system, the objective lens of the accesshead is connected to a feedback mechanism which keeps the access head infocus with respect to the spinning and imperfectly flat optical disk.The signal which controls the feedback mechanism is often created by aphotodiode array. Between the optical disk and the photo array is placedan astigmatic lens. As reflected light from the disk passes through thisastigmatic lens, a system which is out of focus will create a light spotwhich is elliptical on the photo array. By sensing which direction thelight is elliptically elongated, the photo array creates the appropriatesignal to move the access head further from or closer to the opticaldisk, thus bringing it into focus.

In systems having more than one information-carrying layer, it isnecessary not only to maintain focus on the current information layer,but to configure the access head to be able to selectively focus on anyof the layers. For example, in a dual layer disk, the head will need tofocus sometimes on the first layer and sometimes on the second layer.Astigmatic focusing has been adopted to assist with this need. Forexample, U.S. Pat. No. 5,811,789 to Nix provides an invention whichselectively focuses a head on the desired data layer of an multi-layeroptical medium. In the Nix invention, an astigmatic cylindrical lens isplaced in the path of light between the data bearing surface and theaccess head's photodiode array. The photodiode array generates a focuserror signal (“FES”) which is used by a servo assembly to bring the lensinto focus on the appropriate information layer.

However, astigmatic focus is more difficult to accomplish on multi-layeroptical disks than on single layer disks. In a disk have two or morelayers, the information-storing layers of medium are closely spaced withall but the furthest information layer semi-transparent rather thanentirely reflective. In a single layer system, all of the reflectedlight is caused by the reflection from that single layer. In amulti-layer system, when accessing the top information layer, only aportion of the reflected light is caused by a reflection from the toplayer. Some light passes through the top layer to the second (andsubsequent layers). These subsequent layers also reflect a portion ofthe light back to the astigmatic lens. These additional sources ofreflected light keep the read-and-focus photo array detector fromoperating properly, rendering the focus error signal unreliable.

One solution is to use thick spacer layers between disk informationlayers. This introduces spherical aberration resulting in a larger spotand therefore in the end reducing the allowable information density.Another solution is to use an astigmatic detector lens that creates a“steep” FES S-Curve that goes through its peak and valley for lessdefocus motion. This results in a small spot at the quad detector,making the system more sensitive to lateral drifts.

What is needed is a system for a multi-layer optical storage unit whichoffers improved focus signal. Such a system should be economical andeasy to implement without causing a degradation in spot quality at thedisk or in the introduction of complicated assemblies. Ideally, what isneeded is a way to remove the extraneous reflected light so that thephoto array detector can function properly.

SUMMARY OF THE INVENTION

The invention is an auto-focus system for a DVD reader or other opticalaccess unit which reads and/or writes information from/to at least twoinformation layers on an optical storage medium such as a DVD or DVR.The auto-focus system includes an astigmatic lens which creates at leasttwo focal line images. A confocal aperture is placed at the first focalline image created by the astigmatic lens. The placement of the confocalaperture allows extraneous out-of-focus light rays to be successfullyblocked from reaching the photo detector array which together withdetector electronics creates a focus error signal (FES). The FES drivesa servo assembly connected to an access head of the optical access unit,bringing the access head into focus. In some embodiments, the confocalaperture is “bow-tie” shaped, allowing for simplified assembly andeasing in-plane rotational alignment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a prior art astigmatic system for auto-focusing a read/writeaccess head.

FIG. 2 is the present invention, combining an astigmatic auto-focussystem with a confocal aperture.

FIGS. 3A and 3B show one embodiment of a specially shaped confocalaperture in more detail.

FIGS. 4A and 4B illustrate the auto-focusing system of the presentinvention in action.

FIG. 5 shows the sensor assembly for a prior art astigmatic system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following drawings, an attempt has been made to label likeelements with the same element number. Referring to FIG. 1, a prior artastigmatic system for autofocusing a read/write access unit head in amulti-layer optical storage unit is shown. The cross-section of anastigmatic lens 105 is shown in FIG. 1 along its YZ-plane as element105.1 and along its XZ-plane as element 105.2. While the system only hasone astigmatic lens 105, the lens creates two focal line images—one inthe YZ plane 110 and one in the XZ plane—for light rays 120 that arerefracted by the lens 105. The path of light through the lens 105 canform a “circle of confusion” on a receiving surface. In the prior art,for a single layer optical disk, focus is generally set where thediameter of the circle of confusion is minimized—i.e, the circle ofleast confusion. This point appears at a sensor location 125 which isbetween the YZ focal line image 110 and the XZ focal line image 115. Foran astigmatic lens 105 the circle of least confusion 125 is generallymidway between the focal lines images.

FIG. 1 includes a focus detection system 128 which is aligned with thesensor location or circle of least confusion 125, and in which theastigmatic lens is placed in a return beam from the access unit head'sobjective lens. FIG. 5 shows focus detection system 128 in more detail.It includes a photodiode array detector 130 having four quadrants 135.When the lens 105 is properly focused, the light 120 forms a smallcircle of illumination at the center of the photo detector 130. However,when the light 120 is out of focus, it illuminates one or more of thequadrants of the photo detector 135 unevenly. This uneven illuminationis sensed by amplifier 138 to produce a focus error signal (“FES”) 140.The FES 140 is then fed to a servo loop that controls a servo actuator(not shown) which moves the lens 105 closer to or further away from theoptical disk's surface, bringing it into focus.

As has been previously explained, in a dual-layer optical storagesystem, the spacer layers are semi-transparent, allowing light to reacheach of the information layers and then to be reflected back to the lens105. This extraneous, undesired light 122 causes the auto-focusingsystem of FIG. 1 to be unreliable as the photo detector array 130receives both the expected light from the layer being accessed 120 aswell as the undesired extraneous light 122.

The use of confocal apertures is known in the art. For example, confocalapertures are used in confocal microscopes. Such microscopes boast a 40%improvement in resolution as compared to traditional microscopes. Evenmore importantly, confocal microscopes offer improved contrast due tothe elimination of out-of-focus glare.

The present invention removes the undesired extraneous light 122 from anastigmatic focus system by the use of a confocal aperture. By correctlypositioning the confocal aperture, the present invention ensures thatout-of-focus glare from the non-accessed layers is blocked so that thephoto detector array 130 can operate correctly.

FIG. 2 shows the present invention, which is an astigmatic focusingsystem with the addition of a confocal aperture 205 positioned at thefirst focal line image 110 created by the lens. When accessing the firstinformation layer, the aperture 205 in the astigmatic focusing system ofFIG. 2 generally blocks (most of) the light returning from the secondinformation layer and passes all the light returning from the firstinformation layer. When focusing on the second information layer, theaperture 205 generally blocks (most of) the light returning from thefirst information layer 122 and passes all the light returning from thesecond information layer. In both cases light returning from theaddressed information is passed through the aperture 205, while lightreturning from the non-addressed layer is mostly blocked, thus enablingthe generation of an appropriate Focus Error Signal.

In some embodiments the confocal aperture 205 is mounted directly on thephoto detector's cover 128. However, assembly is not easy. The aperture205 must be precisely mounted and aligned in order to properly block themarginal light rays 122. To lessen the burden of installing andmaintaining the aperture 205, some embodiments of the present inventioninclude a specially shaped aperture. Traditionally, confocal aperturesare rectangular slits or small pin holes. FIGS. 3A and 3B illustrate anenhanced confocal aperture which is shaped as a “bow tie.” FIG. 3A showsa front plan view of the bow tie confocal aperture 205, named for itsbow tie shaped opening. This shape eases manufacturing alignment andallows rotational tolerance of the astigmatic line with the aperture. Aswith the FES, in some embodiments, a feedback servo with a correctionsignal can be implemented with the aperture to properly align the bowtie slit in the X and Y plane. In an even more sophisticated correctionsystem, the aperture could be moved along the z-axis to correct fortolerances in the z-direction. These correction systems allow smalleraperture slits, thus enabling to filter out more light of non-accesseddisk layers.

The confocal aperture can be created on a glass plate. One preferredembodiment uses a six by three millimeter rectangular glass plate thatis 1.0 mm +/−0.11 mm thick. In such an embodiment, the aperture is partof a mask applied to the glass plate. A covering, such as paint, isapplied to the glass plate to make it opaque but for the bow-tie shapedaperture, which is left transparent.

FIG. 3B points out the detailed orientation of the bow-tie aperture 205in one preferred embodiment. In such an embodiment, angle 305 ispreferably generally 5 to 15 degrees. In this embodiment, the length ofeach of the halves 315 of the aperture is 0.4 mm +/−0.010 mm while thecentral opening in the aperture 320 is perhaps 0.015 mm +/−0.001 mm. Thebow tie is sometimes placed on an angle (such as 45 degrees, as shown byelement 325). Such an angle 325 is used in some systems to align thecylinder of the astigmatic lens under 45 degrees with the track on theinformation layer. As one skilled in the art will recognize, thesedimensions can be altered to create different confocal apertures withsimilar or enhanced properties.

FIGS. 4A and 4B show the auto-focusing system of the present inventionin action. In FIG. 4A, a cross section of an optical disk is shownhaving a substrate 415 protecting a first information carrying layer405, a spacer layer 420, and a second information carrying layer 410.FIG. 4A illustrates accessing information from the first informationlayer 405 while FIG. 4B illustrates accessing information from thesecond layer 410. In FIG. 4B, while the light rays 120 reflected fromthe accessed layer pass through confocal aperture 205, most of theextraneous light rays 122 reflected from the non-accessed layer aresuccessfully blocked by the aperture 205. In this way, the system canperform better auto-focusing as the circle of light illuminated upon thephoto detector 130 is kept crisp and usable. Without the confocalaperture's assistance enough light 122 is reflected from thenon-accessed layer to cause light to hit the photo detector 130, causingan incorrect focus error signal 140.

The present invention does not require that the forward beam to theobjective lens and the return beam from the objective lens becollimated. Such systems are often called infinite conjugate systems. Adiverging forward path of the beam and converging return beam can alsobe used, which is often called a finite conjugate system. Thus, theincoming objective beam to the astigmatic lens converges. In such acase, the focal line image moves closer to the astigmatic lens,requiring that the aperture be placed closer to the lens.

While the detailed description has addressed a system which accesses anoptical medium having two information layers, the system can be readilyadapted for mediums having more information layers—for example, threelayers or four layers. In such a system, the use of an aperture andastigmatic lens would ensure that the light rays reflected from theaddressed layer reach the photodiode array (or other focusing sensor)while most of the light rays reflected from the non-addressedinformation layers are blocked from reaching the photodiode array.

While the specification describes particular embodiments of the presentinvention, those of ordinary skill can devise variations of the presentinvention without departing from the inventive concept.

1. An auto-focus system of an optical access unit for reading andwriting data from and to a first information layer and a secondinformation layer of an optical storage medium, the system comprising:an astigmatic lens which creates a first set of line images and a secondset of line images, wherein the first set of line images comprises atleast one line image for refracting light rays from the firstinformation layer, and wherein the second set of line images comprisesat least one line image for refracting light rays from the secondinformation layer, and a confocal aperture placed at a first focal lineimage from the first set of line images created by the astigmatic lens,wherein the confocal aperture blocks the light rays reflected from thesecond information layer but not the light rays reflected from the firstinformation layer when accessing the first information layer, andwherein the confocal aperture blocks the light rays reflected from thefirst information layer but not the light rays reflected from the secondinformation layer when accessing the second information layer.
 2. Theauto-focus system of claim 1, wherein the confocal aperture is bow-tieshaped.
 3. The auto-focus system of claim 1, further comprising aquadrant photodiode array placed generally in the circle of leastconfusion for the light rays refracted through the astigmatic lens. 4.The auto-focus system of claim 3, further comprising: a servo assemblyconnected to an access head of the optical access unit, for moving theaccess head to a proper position; wherein the servo assembly iselectrically connected to the quadrant photodiode array.
 5. Theauto-focus system of claim 1, wherein the optical access unit comprisesan objective lens, wherein the astigmatic lens is placed in a returnbeam from the objective lens, and wherein the objective lens is focusedon the first information layer or the second information layer of theoptical storage medium.
 6. The auto-focus system of claim 1, wherein theoptical access unit is a finite conjugate system.
 7. An auto-focussystem of an optical access unit for reading and writing data from andto a plurality of information layers of an optical storage medium havingdata stored on at least a first information layer, a second informationlayer, and a third information layer, the system comprising: anastigmatic lens which creates a first focal line image and a secondfocal line image for refracting light rays reflected from an addressedlayer, wherein one of the information layers is the addressed layer andthe remaining information layers are the non-addressed layers; and aconfocal aperture placed at the first focal line image created by theastigmatic lens, wherein the confocal aperture passes the light raysreflected from the addressed layer, and blocks generally the light raysreflected from the non-addressed layers.
 8. The auto-focus system ofclaim 7, wherein the confocal aperture is bow-tie shaped.
 9. Theauto-focus system of claim 7, further comprising a quadrant photodiodearray placed generally in the circle of least confusion for the lightrays refracted through the astigmatic lens.
 10. The auto-focus system ofclaim 9, further comprising: a servo assembly connected to an accesshead of the optical access unit, for moving the access head to a properposition; wherein the servo assembly is electrically connected to thequadrant photodiode array.
 11. The auto-focus system of claim 7, whereinthe optical access unit comprises an objective lens, wherein theastigmatic lens is placed in a return beam from the objective lens, andwherein the objective lens is focused on the first information layer ofthe optical storage medium.
 12. The auto-focus system of claim 7,wherein the optical access unit is a finite conjugate system.
 13. Anauto-focus system of an optical access unit for reading or writing dataon a plurality of information layers of an optical storage medium,comprising: an astigmatic lens which creates a plurality of focal lineimages for refracting light rays reflected from an addressed layer fromone of the plurality of information layers of the optical storagemedium; and a confocal aperture placed at the one of the plurality offocal line images created by the astigmatic lens, wherein the confocalaperture passes the light rays reflected from the addressed informationlayer, and wherein the confocal aperture blocks generally the light raysreflected from the non-addressed information layers.
 14. The auto-focussystem of claim 13, wherein the confocal aperture is bow-tie shaped. 15.The auto-focus system of claim 13, further comprising a quadrantphotodiode array placed generally in the circle of least confusion forthe light rays refracted through the astigmatic lens.
 16. The auto-focussystem of claim 15, further comprising: a servo assembly connected to anaccess head of the optical access unit, for moving the access head to aproper position; wherein the servo assembly is electrically connected tothe quadrant photodiode array.
 17. The auto-focus system of claim 13,wherein the optical access unit comprises an objective lens, wherein theastigmatic lens is placed in a return beam from the objective lens, andwherein the objective lens is focused on the corresponding informationlayer of the optical storage medium.
 18. The auto-focus system of claim13, wherein the optical access unit is a finite conjugate system.